I do all socks on 2 circulars. I never found a great way of doing the gusset, so I made up my version:
As normal when knitting on 2 circs, you will have 1/2 the stitches on each needle. Pick one needle to be the heel stitches (ignore the other needle for now ).
The heel flap (a slip stitch pattern) and the heel (the knit and purl back and forth with k2T and P2T) are done on one needle.
Picking up the gusset. At this point I grab a couple of extra circular needles, because I find there are just too many stitches to fit on the 2 needles.
First side: Pick up stitches - one for each slip stitch and one or two extra at the end to avoid holes. I put these stitches on one of the new needles. Knit across the needle that has been just sitting there. Pick up stitches on the other side of the slip stitches, trying to get the same number as you picked up on the other side. These also go on an extra needle. Now you can knit across the stitches that you turned the heel on. As you do the decrease stitches to get back to the number you originally cast-on or the number specified by your pattern, you can drop out the extra needles. I usually put a stitch marker at the middle of the heel stitches so that I can count the stitches on each side as I decrease to make sure I don't miss a decrease on one side.
When you are back to the number you need for the foot, just continue on 2 needles.
I had a really hard time with heels until I bought I few extra needles from Knit Picks. I had been using Addi Turbo's but the Knit Picks are much more affordable.
Hope this helps. If you still have questions, feel free to message me here or on Ravelry where I am cinnamonsheep.
Klaralund sweater in Kureyon